Russia has said for decades that they see Ukraine joining NATO as an existential threat to their country. It doesn't matter what we say or what we believe. Clearly they see it this way.

I doubt that Russia seriously believes they could take on NATO and win. Which is why they are not attacking NATO, and invaded Ukraine (with reason) prior to Ukraine becoming officially a NATO member (they were already defacto NATO).

I think it is more of a situation that Russia said, “Either we stop the expansion of NATO into Ukraine now, or we will be destroyed later.” We (the West / NATO) put them in this situation by not taking Russia's clearly stated red lines seriously.